What is space? What is time?
If you are looking for a modern physicist’s take on the very best answers to those two questions, and all of the related mysteries they stir up, check out Brian Greene’s Fabric of the Cosmos. 
Greene has that rare and shining combination of a precise mathematical mind (that can earn an undergraduate degree from Harvard, a doctorate from Oxford, and teach math and physics at such universities as Cornell and Columbia,) along with the infectious enthusiasm of your 8-year old nephew when he is showing you his latest bug collection, or some new fact of astronomy he has just learned.
With Section headings like “I. Reality’s Arena,” “II. Time and Experience,” “III. Spacetime and Cosmology,” “V. Reality and Imagination,” Greene demonstrates his commitment not only to covering the most relevant and central issues in the modern physical sciences, but to doing so in an engaging and interesting way, by disdaining to discuss anything too mathematically sticky, and persistantly wooing the non-specialist.
Chapters 1-3 are a well-paced recapitulation of the question-hypothesis-antithesis-synthesis process regarding questions about space. Is space an entity of itself, or is it merely a name we use to describe the fact that things are spatially separated from each other? Newton says space exists, independantly and absolutely. Leibniz says it does not; space without things in it is meaningless. Ernst Mach agrees, with qualification. Einstein shows up and discovers that space and time are so inextricably entagled that it is difficult to tell them apart. In fact, they are not apart. “They” are one thing: spacetime. Greene fully explores what this discovery might mean for space, gravity, light, and time.
In Chapter 1 he also gives an interesting take on how the scientific (and, I would call it, dialectical) process works, tipping his hand with regard to the answer of “Why does it even matter?” He responsibly considers the possibility that the “scientific enterprise” is just another futile circle, like the curse of Sisyphus. He provides a passionate rejoinder against this anology, assuring himself and readers that progress is indeed beinge made, and calling us to fight forward!
Chapters 5-7 are a similary robust and interesting romp through the oldest and latest theories about time, with special attention given to Einstein and the satisfying solutions he presented, along with the new problems he posed.
Chapters 8-11 (on “Origins & Cosmology”) are perhaps the least compelling. His account of the origion of the world is thoroughly and dogmatically Naturalistic. I would guess that it is the relative laxity of scientists subject to this intellectual stronghold that lets them produce dramatically less rigourous arguments and theories, and encourages them to give these theories inordinate consideration.
Chapters 12 and 13 launch us into the bewildering and fascinating world of String Theory (or Superstring Theory) and its latest mutation, M-Theory. Greene very excited about the possibilities of String Theory, and where the conversation might be headed, and he guides us through these possibilities with kindness and competence. For someone interested in string theory at an introductory level, flipping through these chapters is the ideal starting point.
Chapters 14-16, as the name (Reality & Imagination) might suggest, are such stuff as science fiction dreams are made of. These chapters are somewhat less academic and understandibly less focused ponderings on related issues about space and time. For instance, there is a chapter-long foray into issues of time travel… Whether or not it will ever be possible to travel into the past, what methods are the most promising, etc. He concludes with educated musings on the future of scientific inquiry in general and string/M-theory in particular, successfully concluding this informative and enjoyable read.
